The present invention relates to a fiber-reinforced composite structure, and, more specifically, to a curing method for the same.
Carbon fiber-reinforced plastics are usually referred to as carbon fiber-reinforced composite materials or structures, which have been frequently applied in various industries including sport articles, aircraft industry, space industry, automobile industry and so on in view of their superior properties such as a higher strength, a higher elastic modulus, a light weight and the like. In the case where a curing resin is used as a matrix resin, there have been usually used such heat-curable resins as an epoxy resin, an unsaturated polyester resin, an acrylic resin, a phenol resin and the like. Accordingly, it is required for curing to heat at an elevated temperature over a prolonged period of time, for example, as taught in JP60-16899B, and there has been also required such a large-scale curing equipment as an oven and the like. Then, there have been suggested for a reduced curing period the heating by microwave, for example, as taught in JP2-182438A or JP3-182309A and the high frequency heating, for example, as taught in JP3-182310A. However, these heating procedures have also required a large-scale equipment. Also, there has been suggested the curing by the heat produced with electric charge to the carbon fiber in a sheet, for example, as disclosed in JP4-158025A and JP5-330482A. However, this process has been difficult to find and fix adequate conditions, as the carbon fiber could be burnt off when transmitted too much or a sufficient curing could not be achieved when transmitted at a low level. Moreover, there has been present the problem, when a sheet with a large area is to be cured, to supply an electric current at a large capacity, which also leads to a further need for the equipment therefor. Also, there has been suggested the process of filament winding wherein a mandrel is supplied therein with, e.g., quick lime and water and a resin is cured by the chemical reaction heat thus produced as disclosed in JP4-224929A; however, this process has the problems in that an applicable form is restricted and that temperature controlling is difficult in view of the chemical reaction heat to be utilized.
On the other hand, a photocurable resin has been known as a one pack type resin curable at room temperature in a short period of time. As the fiber for a fiber-reinforced plastic using this resin as a matrix resin, there has been mainly employed a glass fiber in view of its transmission to active light, transparency and others. For instance, JP53-98290A, JP58-101119A, JP62-44259A and others disclose the technique about the production of a fiber-reinforced plastic using a photocurable resin, wherein a glass fiber solely is employed as a reinforcing fiber. A similar technique is also disclosed in JP1-223953A, JP2-85205A, JP3-104621A, JP3-111429A and others, which state that such opaque reinforcing fiber as carbon fiber would not be preferable when a photocurable resin is used as a matrix resin and could be applied as far as it does not inhibit light transmission. JP63-186774 A discloses a carbon fiber, in addition to a glass fiber, as an applicable reinforcing fiber. However, when a carbon fiber is used as a reinforcing fiber according to this procedure, the cure could be achieved only at the surface of the composite structure and the interior of the composite structure remains uncured. On the other hand, it is suggested as the technique for using a carbon fiber as a reinforcing fiber that an ultraviolet-curable resin having incorporated therein a curing catalyst, e.g., a peroxide and the like is employed and cured only at the surface by ultraviolet irradiation while the interior is cured separately by heat. Such a technique involves individually the step for surface cure and that for interior cure, which results in a reduction on the merit using the photocurable resin capable of being cured in a short period of time. Apart from the above-mentioned techniques, JP4-93843A discloses the technique for a heat-photocurable composition and formation of polymerized image using the same, which is directed to the technique wherein a radical-polymerizable resin composition having incorporated therein a radical-polymerization initiator and a thermally-decomposing curing promoter is irradiated with heat-emitting active radiation and active radiation capable of activating active components solely to form polymerized image. However, it does not disclose any production of a carbon fiber-reinforced plastic, and further it requires irradiation with two types of active radiations together with the thermally-decomposing curing catalyst to be added, which results in complicated procedures.
It is the object of the present invention to provide a carbon fiber composite structure, particularly a prepreg of the carbon fiber fabric such as a woven or nitted fabric impregnated with a resin, without the problems as described above.